Among power conditioning systems with a fuel cell, a power conditioning system is known which can supply output power of a fuel cell to a load by supplying fuel gas (e.g. hydrogen) and oxidant gas (e.g. air) to the fuel cell according to a request of the load connected to the fuel cell.
In the power conditioning system as described above, to control an operating state of the fuel cell, alternating-current components of an output current and an output voltage of the fuel cell are measured while an alternating-current voltage signal is output, and an internal impedance of the fuel cell is estimated by computing these measured alternating-current components.
JP4821187B discloses a fuel cell system with a battery (high-voltage secondary battery), a fuel cell provided electrically in parallel to the battery, a DC/DC converter provided on an output side of the battery and an inverter provided between this DC/DC converter and a motor serving as a load.
In this fuel cell system (single converter type), to estimate an internal impedance of the fuel cell, an output target voltage of the DC/DC converter superimposed with an impedance measurement signal (alternating-current voltage signal) is output and an amplitude of the impedance measurement signal after passage through the DC/DC converter is measured. By applying a necessary computation process to this measurement result, the internal impedance of the fuel cell is obtained.
The present applicant has proposed a power conditioning system (twin converter type) with a fuel cell, a battery, and DC/DC converters provided on each of output sides of both the fuel cell and the battery.
In this case, unlike the above single converter type, an internal impedance of the fuel cell can be also measured by outputting an impedance measurement signal as described above to the DC/DC converter on the fuel cell side.